Plasma optical emission spectrometry and plasma mass spectrometry are well known techniques for the analysis of trace elemental concentrations in liquids. The plasma may be formed using microwaves or by using an inductively-coupled system, both being examples of atmospheric-pressure plasmas. Typically detection limits extend to ppb levels for ICP-OES and ppt levels for ICP-MS. AA and AFS are also well known techniques utilizing a flame for atomizing sample material and all these techniques are often used in conjunction with one another in the laboratory.
In all these types of spectrometer, liquid samples are usually diluted and aspirated using a nebulizer or some form of droplet generator, being propelled out of an outlet of the nebulizer or droplet generator into an inert or flame gas stream which carries droplets of the sample-containing liquid into a torch and thereby into the plasma or flame. There are many different designs of torch, but as an example, in ICP-OES and ICP-MS the torch usually comprises three open-ended concentric cylindrical glass tubes along which inert gas streams are fed, the gas usually being argon. The liquid droplets are passed into the upstream end of the innermost cylinder of the torch, whilst additional inert gas is fed into the outer two glass tubes. At the downstream end of the tubes, a coil surrounds the torch and RF current typically at 27 MHz or 40 MHz is driven through the coil. A plasma discharge is initiated in the argon gas within the plasma torch in the vicinity of the coil. The sample-containing droplets are carried along the central channel of the torch and into the plasma discharge. The plasma discharge is sufficiently hot to cause the droplets of liquid entering the plasma to be progressively vaporized, atomized and then partially ionized. Atomized and ionized material is excited and relaxes to emit characteristic wavelengths of light detected by the optical spectrometer in the case of ICP-OES, and ionized material is directed into a vacuum system, through an ion optical system and a mass analyser in the case of ICP-MS. Similar torch arrangements are used in MIP spectrometry; AA and AFS use somewhat differently designed torches.
Typically the sample-containing liquid is formed into a stream of droplets using a nebulizer utilizing a stream of argon gas. Nebulizers produce droplets with a wide range of sizes. However both plasmas and flames are inefficient at dissociating large droplets and these are usually excluded by the use of a spray chamber placed between the nebulizer and the torch. The spray chamber filters the stream of droplets by causing the flow to follow a tortuous path such that the larger droplets impinge upon surfaces in the spray chamber and are drained away, smaller droplets being carried by the flow of gas into the torch. In the cases of ICP-OES and ICP-MS it is well known that only 1-2% of the nebulized sample-containing liquid is in the form of sufficiently small droplets suitable for processing within the torch, and that this form of sample introduction is therefore inefficient.
Various different types of droplet generators have been investigated in order to overcome this problem, and in order to facilitate the analysis of very small volumes of sample solution. Early attempts were made to create a single-droplet generator for flame analytical spectrometry utilizing a continuous fluid jet micro-droplet generator by G. M. Hieftje and H. V. Malmastadt (Analytical Chemistry, Vol. 40, pp. 1860-1867, 1968). Later a vibrating orifice monodisperse aerosol generator was used for investigation of airborne particles by ICP-OES and ICP-MS (H. Kawaguchi et al., Spectrochimica Acta, vol. 41B, pp. 1277-1286, 1986, T. Nomizu et al., Journal of Analytical Atomic Spectrometry, vol. 17, pp. 592-595, 2002).
The ability to produce droplets one at a time and thereby more completely control the droplet ejection process—so-called “droplet-on-demand” techniques—have long been seen as desirable. An early generator with this capability designed principally for inkjet printing was a piezoelectrical droplet generator (U.S. Pat. No. 3,683,212). With this generator no pressurized liquid supply is necessary (though pressurization may be used), and the time at which droplets are ejected together with the size of the droplets may be controlled by the application of an electrical pulse to the piezoelectric element. Such a droplet generator was employed to create a stream of droplets containing sample material, the droplets being passed through an oven so as to make the droplet evaporate to complete or partial dryness before injection into an ICP in order to study oxide ion formation (J. B. French, B. Etkin, R. Jong, Analytical Chemistry, Vol. 66, pp. 685-691, 1994). This coupling of the piezoelectric droplet generator and oven was termed the monodisperse dried microparticulate injector (MDMI) and such systems have been used in other studies (J. W. Olesik and S. E. Hobbs, Analytical Chemistry, vol. 66, pp. 3371-3378, 1994; A. C. Lazar and P. B. Farnsworth, Applied Spectroscopy, vol. 53, pp. 457-470, 1999; A. C. Lazar and P. B. Farnsworth, Applied Spectroscopy, vol. 51, pp. 617-624, 1997).
Use of the piezoelectric droplet generator without the desolvation in an oven has been successfully implemented as a sub-nanoliter sample introduction technique for Laser-Induced Breakdown Spectroscopy and Inductively Coupled Plasma Spectrometry (S. Groh et al., Analytical Chemistry, vol. 82, pp. 2568-2573, 2010; A. Murtazin et al., Spectrochimica Acta, vol. 67B, pp. 3-16, 2012).
Investigations of the use of thermal inkjet droplet generators for use as a sample injector in ICP spectrometry have also been performed (J. O. Orlandini v. Niessen et al., Journal of Analytical Atomic Spectrometry, vol. 26, pp. 1781-1789, 2011).
Micro-dispensers such as these have been used for fundamental studies relating to processes in plasmas, and for the purposes of improving sample utilization and control over sample introduction, but relatively little has been written concerning sample throughput.
Due to the increasingly routine use of spectrometry, sample throughput has become one of the most important requirements as it is this which ultimately determines the cost-per-analysis in routine applications. With the increased sensitivity of instrumentation and automated sample handling, sample throughput is largely limited not by the sample introduction or analysis time but rather by memory effects caused by deposition of material from the previous sample on components of the sample introduction system and spectrometer. Due to the increased sensitivity of the spectrometers and their ultimate detection limits, material deposited upon the sample introduction system is gradually washed away during a “wash” cycle and typically at least 40-60 seconds is needed after each sample to reduce memory effects below an acceptable threshold. It is known that deposition of sample material upon the surfaces of the glassware of the torch, the nebuliser, the spray chamber and the sample line are of particular importance. Direct injection nebulizers were used to reduce memory effects as they eliminated some of these components but they produced aerosols with droplets with very large ranges in size and density and thus compromised performance of the technique (WO9934400A1, WO2005062883A2, WO2005079218A2, US2006087651 A1, US2007299561 A1).
The reduction of dead volumes within a micro-dispenser was considered an important need by Groh et. al. (Anal. Chem., V. 82, No. 6, pp 2568-2573, 2010) and interfacing a monodisperse droplet generator to a ‘lab-on-a-chip’ was suggested, in order to address washout issues and to enable analysis of ultra-low sample volumes.
A 0.1 mL reservoir was utilised by Orlandini (J. Anal. At. Spectrom., 2011, 26, 1781-1789). Filling and rinsing of the reservoir was performed manually using micro-pipettes. A rinsing cycle repeated five times using deionised water was found necessary to regain background signal level after applying 1 mgL−1 sample concentrations with a multi-nozzle inkjet cartridge. Sample solution came in contact with metallic and non-metallic components and sample contamination and elevated background levels were of concern.
Washout times in excess of 30 s were expected in a MDMI system, but due to diffusion and mixing, 100 s washout times were observed to reduce a background by 99%, which was considered long compared to other nebulisers (Lazar and Farnsworth, Applied Spectroscopy, V. 51, No. 5, pp 617-624, 1997).
Against this background, the present invention has been made.